Q: What is the infrared cut filter, what role does it play in a CCTV camera and do I need a day/night camera? I’ve heard of cameras that don’t need a cut filter but handle this task in software. Which is better?
A: Unlike the human eye, camera sensors can see infrared during the day – this impacts on the appearance of colour – green plants turn pink, for instance. In order to match the appearance of human vision, the cut filter only allows the visible light spectrum to pass, reflecting infrared using coatings that reflect or absorb IR wavelengths.
With the CMOS sensors prevalent today, reflective filters are best. These are typically short-pass interference types that bounce IR wavelengths exceptionally well. At night, however, you’re going to want to sense every photon you can at whatever wavelength. Given the camera has gone into monochromatic night mode, pulling the reflective IR cut filter out of the way allows high efficiency sensing of available light, as well as the ability to leverage integrated or adjacent IR light sources.
As you say, some IP cameras undertake night mode in software – they have a fixed cut filter that handles IR in day mode and at night their processors combine red, blue and green pixel signals into white light. The results are 2-fold. A brighter monochromatic image in low light (brighter than the colour image) and a less expensive build. Frankly, I would be wary of software-based night mode. You can only combine red, blue and green if your sensor is discerning red, blue and green and at under 10 lux with a low grade sensor, there’s not going to be much discerning of anything at all.
What is going to be happening is that the digital brightening (amplification) taking place as the barely existent colour pixel signals exit the sensor is the generation of lots and lots of noise by the camera's processing engine. Brightening by adding noise can provide more detail by increasing regional contrast but only if the detail is there to begin with and only to a certain point. Other things to consider are that as noise increases recording sizes blow out and that software-based day/night cameras are not sensitive to IR, so you lose that as well.
We’d be buying quality IR-supported day/night cameras for use in sub-10 lux. After all, there’s no point installing a solution that doesn’t meet your client’s operational specifications, no matter how cheap it is. ♦
